Abstract :
In the first part of this paper the economic aspects of system reliability are explored. The concept of the ´adequate system´ is introduced, which system is defined as the lowest total-cost system that will perform as required. The relation of component-part cost to failure-rate levels is investigated, and the reactions of component performance upon design of an adequate system are examined in terms of the economic factors involved. A typical system is used to illustrate the points developed. Further, the number of systems needed to perform a given mission are considered in terms of annual cost and reliability levels of an individual system. The second part of the paper is concerned with the appraisal of the reliability of a system. This appraisal begins as early as possible in system evolution and makes use of estimation procedures which draw upon prior knowledge. Various assumptions which enter into such estimation methods are examined. It is pointed out that reliability estimates derived analytically must be verified by suitable tests of models at appropriate stages in the development of the system. The value of failure data obtained during the course of system development, manufacture and use in establishing levels of system reliability is emphasised. The final part of the paper deals with the effects of operating environment upon system reliability. The necessity of knowing the environment, and of conducting tests on models to evaluate its effects, is stressed. Two examples are given to illustrate some of the factors associated with environmental testing. One relates to the electrical performance of nickel-cadmium cells under space-simulation conditions; the other to the mechanical response of a structure under missile-flight environment. Broadly, the point of view taken is that electronic-system reliability is a synthesis of many related and interacting factors. Throughout the evolution of system design these factors should be most carefully balanced so that- - the ultimate product will perform at a level of reliability which is the optimum for the time, effort and money expended.
